What are ACLs?
As you have learned so far, routers are used to forward traffic between different networks. As a packet enters an inbound interface of a router, the operating system has to read the Layer 3 header information, such as the source and destination IP addresses, and check the routing table for a suitable route. Once a route has been found, the router forwards the packet through an outbound interface to its destination. Ensuring that all users are able to send and receive messages is excellent in terms of connectivity, but what do security and the restriction of traffic flow between certain networks mean?
The Cisco IOS router has many amazing features and can perform a variety of roles on a network. One such feature is to perform traffic filtering between networks. This is done using a very special method that firewall appliances use to filter traffic, known as an ACL.
Important note
Firewall appliances use a variety of methods to filter inbound and outbound traffic...
